Machines, for example wheel loaders, motor graders, excavators, and dozers are commonly used in material moving applications such as mining, road maintenance, and surface contouring. To effectively accomplish tasks associated with these applications, the machines are typically outfitted with steering components such as hydraulically-powered articulation joints and/or traction devices, and hydraulically powered implements such as shovels, buckets, and blades. A prime mover, for example a diesel, gasoline, or gaseous fuel-powered engine, drives dedicated steering and implement pumps that provide hydraulic power to the steering components and the implements.
The steering pump can be driven by the prime mover to pressurize fluid in response to a fluid demand from steering actuators during a steering event. When no fluid demand exists or when the fluid demand is relatively low, the steering pump may have excess capacity to pressurize fluid.
Hydraulically powered implements are typically velocity controlled based on an actuation position of an operator interface device. For example, an operator interface device such as a joystick, a pedal, or any other suitable operator interface device is movable to generate a signal indicative of a desired velocity of an associated hydraulic actuator. When an operator moves the interface device, the operator expects the hydraulic actuator to move the implement at an associated velocity. However, when multiple actuators are simultaneously operated, the hydraulic fluid flow from a single implement pump can be insufficient to move all of the actuators at their desired velocities. Situations also exist where the single implement pump is undersized and the desired velocity of a single actuator requires a fluid flow rate that exceeds the flow capacity of the single implement pump.
When the steering pump has excess capacity and the implement pump has insufficient capacity to supply a commanded/demanded flow of pressurized fluid, it may be desirable to share pressurized fluid between steering and implement circuits. One example of a system that provides for flow sharing between steering and implement circuits is described in U.S. Pat. No. 4,422,290 (the '290 patent) issued to Huffman on Dec. 27, 1983. The '290 patent describes a hydraulic control system that includes a pressure compensator for diverting excess fluid from a steering pump to an implement control valve based on a fluid pressure within a steering circuit. A solenoid-operated control valve is provided between the pressure compensator and the implement control valve for selectively allowing or blocking fluid flow between the steering circuit and an implement circuit.
Although potentially helpful in some situations, the hydraulic control system of the '290 patent may be problematic. In particular, the disclosed system provides no control for variable displacement type pumps (load-sense or electro-hydraulic pumps), and provides excess fluid flow from the steering pump to the implement circuit regardless of a demand for pressurized fluid flow in the implement circuit. For these reasons, the hydraulic control system of the '290 patent may be inefficient and have limited applicability.
The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.